The present invention relates to an apparatus and method for automatically wave soldering workpieces such as printed circuit boards.
A usual automatic wave soldering apparatus includes a pair of endless chain conveyors driven to advance a printed circuit board at a constant speed from the entrance to exit ends of the apparatus. With the printed circuit board held by gripping fingers, the board is first carried to a fluxer where a foam or spray of flux is applied to the underside of the board. The printed circuit board is then carried over preheaters where the temperature of each board is elevated to approximately 110xc2x0 C. to 130xc2x0 C. so as to evaporate excess flux solvent, activate the flux and minimize thermal shock to the printed circuit board. After the printed circuit board is brought to such a preheat temperature, the board passes over a solder reservoir to receive solder. The board is finally transported to a cool down zone where the solder is cooled to solidify.
Typically, two solder waves are established in the solder reservoir. One of the two solder waves is a turbulent wave, and the other is a non-turbulent wave. The turbulent wave allows the solder to reach component terminations and other hard to reach areas to be soldered, but bridges and icicles of solder remain on the underside of a printed circuit board and can cause a short circuit or damage to sensitive electrical components. Such bridges and icicles can be removed in the subsequent non-turbulent wave.
Various attempts have been made to promote turbulence. For example, Japanese patent publication No. 62-46270 discloses a plurality of vanes disposed in a solder nozzle and interconnected by a rod. The rod and thus, the vanes are rotated or reciprocated by means of a motor to cause molten solder to flow in a turbulence wave. Japanese patent publication No. 62-35857 discloses a cylindrical body disposed in the upper end of a solder nozzle. A multiplicity of openings are defined in the cylindrical body. While the cylindrical body is rotated by a motor, molten solder is forced through the openings to thereby promote turbulence in a solder wave. A disadvantage with these designs is that the rotational speed of the motor fluctuates due to heat degenerated in a solder reservoir in which the motor is disposed. The heat also reduces the service life of the motor. Another problem is that the vanes and other movable turbulent means tend to cause molten solder to spatter. Such spattering creates bridges, particularly between closely adjacent conductors.
Japanese laid-open patent publication NO. 58-178593 discloses two waves established in two respective solder nozzles without the use of vanes and other movable turbulent means. The solder waves are both smooth or laminar waves. It has been found that such laminar waves are unable to cause molten solder to fill through holes through which leads of electronic components extend.
Accordingly, it is an object of the present invention to provide an automatic wave soldering apparatus and method which promote turbulence in a solder wave without the use of a motor or similar means and which allow molten solder to fill through holes and reach component terminations and other hard to reach areas to be soldered, regardless of orientations of selected areas of electronic components to be soldered.
According to one aspect of the present invention, there is provided an apparatus for wave soldering a printed circuit board, which comprises a solder reservoir for holding a supply of molten solder, a solder nozzle disposed in the solder reservoir to establish a solder wave, and means for promoting turbulence in the solder wave and for increasing flow velocity of the molten solder. The turbulent means is disposed in the solder nozzle to thereby provide at least one upwardly inclined outlet through which the molten solder emerges at an angle relative to the direction of movement of the printed circuit board.
In a preferred embodiment, the turbulent means includes a trapezoidal element secured within the solder nozzle. The trapezoidal element has horizontal top and bottom sides. This bottom side acts as a step to cause molten solder to flow in a turbulent wave. The trapezoidal element is fixed in position and thus, will not cause molten solder to spatter. Also, this arrangement eliminates the need for a motor to create turbulence. The solder nozzle is composed of a pair of upstream and downstream vertical walls, and a pair of opposite side walls connected to the vertical walls. Preferably, the downstream vertical wall has an inclined upper end. The inclined upper end of the vertical wall and the trapezoidal element collectively reduce the sectional area of the solder nozzle to thereby increase the flow velocity of the molten solder.
In a preferred embodiment, the turbulent means includes a baffle disposed between the upstream and downstream vertical walls to form an upwardly inclined upstream outlet between the baffle and the upstream vertical wall and an upwardly inclined downstream outlet between the baffle and the downstream vertical wall, and a pair of valves disposed on opposite sides of the baffle and adapted to selectively open and close the upstream and downstream outlets. The upwardly inclined upstream outlet is inclined at an acute angle against the direction of movement of the printed circuit board, whereas the upwardly inclined downstream outlet is inclined at an acute angle in the direction of movement of the printed circuit board. This arrangement allows molten solder to reach all areas to be soldered, regardless of the orientations of electrical components on a printed circuit board.
According to another aspect of the present invention, there is provided a method of wave soldering a printed circuit board, which comprises selectively establishing at least either one of a turbulent wave which flows counter to the direction of movement of the printed circuit board, and a turbulent wave which flows in the direction of movement of the printed circuit board, establishing a substantially non-turbulent solder wave, and passing the printed circuit board along a predetermined path to allow the printed circuit board to successively engage at least a selected one of the turbulent waves and the non-turbulent wave.